Control apparatus



Patented Dec. 8, 1942 UNITED STATES PATENT OFF ICE CONTROL APPARATUSWilliam L- McGrath, Philadelphia, la., assignor Minneapolis-HoneywellRegulator Company, Minneapolis, Minn, a corporation of DelawareApplication June 16, 1941, Serial No. 398,308

(01. zas-vo) Claims.

object of the invention is to provide a control system operated inaccordance with the relative positions of galvanometer pointers whichare responsive to the controlling conditions.

A further object of the invention is to construct a control systemresponsive to the unbalance of a Wheatstone bridge circuit including acondition responsive resistance, wherein means responsive to a limitingcondition is provided and control of 'the system is shifted from saidbridge circuit to said limiting condition responsive means when thelimiting condition exceeds a predetermined value. A further object is toprovide such a system in which the limiting condition responsive meansis another bridge circuit.

Another object of the invention is to provide an improvedgalvanometer-operated relay mechanism of the clamp-the-pointer type. Afurther object is to construct such a mechanism wherein two galvanometerpointers are simultaneously clamped, and one or the other of thepointers selectively control the relay mechanism. A further object is toprovide such a mechanism wherein the pointer which controls the relay isselected in accordance with the relative positions of the two pointers.

A further object of the invention is to provide a control system whichis selectively responsive to one of two or more conditions, whereinmeans is provided for shifting control from one condition to another inaccordance with the relative values of the conditions.

In condition control systems, and particularly in temperature controlsystems, it has long been known that a Wheatstone bridge circuitincluding a temperature responsive resistance element provided a verysensitive means for obtaining an indication of temperature changes. Ithas been desired to build a temperature control system including aWheatstone bridge having a temperature responsive resistance as thesensitive element, and including a floating limit control device. By a,floating limit control device is meant a device responsive to a limitingcondition which acts to limit increase of the heat supply as thelimiting condition approaches a predetermined value, and

which acts, to decrease the supply it the limiting condition exceeds thepredetermined value. The application of floating limit control toWheatstone bridge circuits in the past has involved the use of some kindof rheostat operated by a temperature sensitive element. Such rheostatshave been generally unsatisfactory because of the irregular resistancevariation due to the low contact pressures necessary when the contactsare moved by low-powered temperature sensitive elements. The use ofcondition responsive elements, such as temperature responsive resistors,for limit controls in bridge circuits has been considered undesirablebecause their efiect on the bridge circuit was always present, unlessswitching means were provided for disconnecting them at predeterminedtimes. When no such switching means was used, the limiting conditioncaused bad unbalancing effects on the system due to its variations whenit was not near its limiting value. 0n, the other hand, when switchingmeans was used, the irregular variations of the contact resistancecaused trouble.

It is therefore a further object of this invention to construct acontrol system in which the principal condition responsive element is atemperature responsive resistance, and in which a floating limit controlis obtained from a second temperature responsive resistance.

Other objects and advantages of the present invention will becomeapparent from a considera tion of the attached specification, claims,and drawing.

The single figure of the drawing shows a diagrammatic view of a controlsystem embodying my invention.

In the drawing, Ill represents a space which is heated by a furnace i I,shown, by way of example, as being of the hot air type. Air is deliveredfrom the furnace to the space i0 through an outlet duct I2.

The furnace II is provided with a fluid fuel burner II, to which fuel issupplied through a pipe I provided with a valve it. The valve I5 isoperated by a driving mechanism generally indicated at 16 which includesanelectricai motor having a pair of windings I1 and i8. It should bereadily understood that the present illustration is by way of exampleonly, and that the present invention is equally adaptable to other typesof furnaces burning diflerent kinds of fuel. The windings l1 and 18 areso related to the operating mechanism Ii that they drive the valve inopposite directions. For example, if energization of the winding l1causes movement or the valve in opening direction, then energisation o!the winding I4 causes movement of the valve in closing direction.

A bridge circuit 24 is provided having input terminals 2I and 22, andoutput terminals 23 and 24. In the upper right-hand arm of the bridge 24is a resistance element 25 having an appreciable temperature coeilicientof resistance and exposed to the temperature in the space I 4.

The upper left arm of the bridge, which connects terminals 2| and 23,contains a flxed resistance 24. The lower left arm of the bridge circuit24, which connects input terminal 22 with output terminal 23, contains afixed resistance 34 and a variable resistance 3I. By adjustment ofresistance 3I, the temperature at which bridge 24 is balanced may befixed at any desired value. The lower right arm of the bridge 24, whichconnects input terminal 22 with output terminal 24, contains a fixedresistance 32.

Energy is supplied to the bridge circuit 24 from a suitable source,shown in the drawing as a battery 33, connected to the input terminals2I and 22 through conductors 34 and 35, respectively.

A galvanometer 34 is provided, having a moving coil 31, connected tooutput terminals 23 and 24 of bridge circuit 24 through conductors 44and 4 I, respectively. The galvanometer 36 has a pointer 42, normallybiased to a central position, which may be the position shown in thedrawing, and deflectable from that position by a current flowing throughthe coil 31. The pointer 42 indicates the unbalance potential of thebridge circuit 24, and thereby the departure of the temperature to whichthe element 25 is exposed from a value predetermined by the setting ofthe variable resistance 3i.

A bridge circuit 43 has input terminals 44 and 45 and output terminals44 and 41. The input terminals 44 and 45 are connected to the samesource of electrical energy which supplies bridge circuit 24, that is,the battery 33, by conductors 54 and 34, and conductors 5| and 35,respectively.

The upper right arm of the bridge circuit 43, which connects inputterminal 44 with output terminal 41, contains an element 52 having anappreciable temperature coefllcient of resistance. The element 52 isexposed to the temperature of the air in the discharge duct I2 near thepoint where it leaves the furnace I I.

The upper left arm of the bridge circuit 43, which connects inputterminal 44 with output terminal 46, contains a fixed resistance 53. Thelower left arm of the bridge circuit 43, which connects input terminal45 with output terminal 46, contains a fixed resistance 54 and avariable resistance 55. The function of variable resistance 55 is todetermine the temperature at which the bridge 43 is baianced. The lowerright arm of the bridge circuit 43, which connects input terminal 45with output terminal 41, contains a fixed resistance 55.

A galvonometer 51 has a moving coil 64, which is connected to outputterminals 46 and 41 by conductors 5I and 62, respectively. The movingcoil 64 operates a pointer 63. The pointer 63 is deflected from acentral position to which it is biased in accordance with the unbalancepotential of bridge circuit 43. This unbalance potential is proportionalto the variation of the temperature to which element 52 is exposed froma predetermined value which is fixed by the setting of the variableresistance 55.

Energy is supplied to the windings I1 and I4 of the driving mechanism I6from power supply lines 44 and 45 through a transformer 44 having aprimary winding 41 and a secondary winding 44. The energization oiwindings l1 and I4 is controlled by a sensitive relay mechanismgenerally indicated at 14. which is driven by a constan ly rotatingmotor 1I having a winding 12 connected to power lines 44 and 44 throughconductors 13 and 14, respectively. The motor 1| drives. throughsuitable mechanical connections (not shown) series of three cams, 14, 14and 11.

The cam 15 drives, through a follower 44, and a strain release spring4|, a switch positioner arm 42. The arm 42 is pivoted for rotation aboutits lower extremity, as at 43. A spring 44 biases the arm 42 forrotation about the pivot 43 in a clockwise direction, therebymaintaining the follower 44 in engagement with the cam 15. A link 44 ispivoted as at 44, to an intermediate point on the arm 42. The link 45 isconnected through a strain release spring 41 to a reciprocablesupporting member 44 which carries on its underside a switch contact 4I.

A pair of plates 42 and 43 are adiustably attached to the upper end ofthe arm 42 by means of screw and slot connections 44 and respectively. Apair of upstruck ears 46 and 41 on the.

are 42 extend through the slots of these connections so as to guide themovement of the plates 42 and 43 in a direction transverse to the lengthof arm 42, when the plates are being adjusted. The plates 42 and 93 aremounted one above the other in such relation that as the arm 42 isreciprocated, plate 92 moves transversely to the path of galvanometerpointer 42, while plate 43 moves transversely to the path of movement ofgalvanometer pointer 63.

The cam 16 operates against a follower I44 on a switch contactsupporting arm I4I which is pivoted at its righthand end I42. The armI4I carries an insulating block I43 on which are mounted three contactsI44, I45 and I46. The ends of the block I43 are provided with upstandingprojections I41 and I44, which extend into the path of movement ofcontact 3| and limit its movement transversely to the contacts I44, I45and I46.

Cam 11 operates a follower link II4, the upper end II I' of which ispivoted to a scissors link I I2. The link H2 is pivoted at II3 toanother scissors link II4, the right-hand end of which is pivoted to astationary support, as at II5, A tension spring II6 joins the right-handend of the scissors links II 2 and H4. The spring II6 tends to cause thescissors links to close toward each other. The left-hand end of scissorslinks H2 and H4 are provided with slots which receive pins attached tothe ends of beater arms I I1 and H4, respectively.

A horizontal bar I24 extends between the galvanometer pointers 42 and43. Operation of the cam 11 acts through the scissors links H2 and H4 toreciprocate the beater arms I I1 and H4 so as to clamp the galvanometerpointers 63 and 42 respectively against the horizontal bar I24.

Operation The operation of the system will first be described as thoughthe galvanometer 51 were omitted and the valve I5 were operated inaccordance with the unbalance of the bridge circuit 24 acting throughthe galvanometer 36.

When the temperature in the space I4 is at the value which the systemhas been set to maintain by adjustment of the resistance 3|, the bridgecircuit 24 is balanced and no current flows through the galvanometercoil 31. The pointer 42 will therefore be in its normal or intermediateposition. The valve I5 is at this time in such a position that the heatsupplied by the furnace II to the space I is just sumcient to balancethe heat loss from the space, and the space is therefore maintained atthe desired temperature.

As the motor II rotates the cam 11 moves in a counterclockwisedirection. This cam 11 has a relatively long riser portion I2I, anabrupt drop-off portion I22, and a low dwell portion I23. When the partsare in the position shown in the drawing, the follower H0 is near thetop of the riser portion 2|, and the beater arm II 8 has been moved awayfrom the pointers 42 and 63. As the drop-off portion I22 passes underthe follower I I0, the latter drops downwardly under the influence ofthe spring II6, thereby closing the scissors and bringing the beater arm8 downwardly to clamp the pointer 42 against the horizontal bar I20. Thepointer 42 remains clamped while the follower H0 is engaging the dwellportion I23 of the cam 'II, When the follower 0 engages the riserportion I2I, the beater arm I I8 is lifted from the pointer 42, and thelatter is again allowed to move freely under the influence of the coil31.

While the follower H0 is engaging the dwell portion I23 and the pointer42 is therefore clamped, the cam I5operates against the follower 80 tomove the arm 82 to the left. This movement of the arm 82 continues untilthe plate 82 engages the pointer 42. When this engagement takes place,further movement of the arm 82 to the left is prevented, and furthermovement of the cam follower 80 in that direction is taken up by thestrain release spring 8|. The cam I5 is provided with a surface portionI24 which is cocentric with respect to the center of rotation of the camI5 and a second portion I25 which is concentric with the center ofrotation of cam 15. The eccentric portion I24 serves to cause recip-'rocation of the arm 82, while the concentric portion I25 maintains thearm 82 at the left-hand end of its range of movement, or in the positionin which it is stopped by engagement of plate 82 with pointer 42.

While the arm 82 is maintained in its stopped position by the cam I5,the cam I6 operates against the follower I00 to lift the switch contactsI04, I05 and I06 so that one or more of them engages the switch contact8|. The position of switch contact 8| at this time depends upon theposition in which the arm 82 has been stopped. If the arm 82 is stoppedin its position corresponding to the center position of pointer 42, thencontact 8| is in its corresponding center position, and upon the liftingof the contacts I04, I05 and I06, only the contact I05 engages thecontact 8|. No circuit is then completed to either winding II or I8 ofthe valve operating mechanism I6, and the setting of the valve I5 isleft undisturbed.

Now let it be assumed that the temperature in the space l0 falls belowthe value which the system has not been set to maintain. If it isconsidered that the element 25 has a positive temperature coefllcient ofresistance, then the drop in temperature of the space I0 will causeterminal 28. A current therefore flows from terminal 24 to terminal 28through conductor 4|, coil 81 and conductor 40.

This current flowingthrough the coil 31 causes the pointer 42 to bedeflected to the left. 0n the next reciprocation of the arm 82, theplate 82 engages the pointer 42 at a point to the left of its centerposition. If the deflection of pointer 42 is large enough, the arm 82 isstopped at a position far enough to the left so that switch contact 8|is in line with both contacts I04 and I05 when they are next raised bythe operation of cam I6. If this is the case, engagement of contact 8|with contacts 04 and I05 completes a circuit through winding I! of valveoperating mechanism I6. This circuit may be traced from the left-handend of transformer secondary winding 68 through a conductor I30, contactI05, contact 8|, contact I04, a conductor I3I, winding I1, and aconductor I32 to the right-hand end of secondary winding 88.Energization of winding II causes motor I6 to drive the valve I5 inopening direction, thereby increasing the supply of heat to the space I0and tending to return the temperature of that space to its desiredvalue. The winding II continues to be intermittently energized byengagement of contact 8| with contacts I04 and I05 as long as thepointer 42 is deflected to a position far enough to the left of itscentral position so that the contact 8| is moved far enough to the leftto engage contact I04.

Now let it be assumed that the temperature in the space I0 rises abovethe value which the system has been set to maintain. This increase intemperature results in an increase in the resistance of the element 25,thereby causing the potential of output terminal 24 to become negativewith respect to that of output terminal 23. A current therefore flowsfrom terminal 23 to terminal 24 through conductor 40, coil 31, andconductor 4|. This current flows through the galvanometer coil 31 in adirection opposite to the current caused by a temperature drop, and

. therefore causes pointer 42 to be deflected to the right of its centerposition. Accordingly, the arm 82 is stopped by engagement of plate 92with pointer 42 at a position to the right of its center position. ,Ifthe deflection of pointer 42 is great enough the contact 8| is stoppedin line with both contacts I05 and I06.

Upon engagement of contact 8| with contacts I05 and I06 an energizingcircuit is completed for winding I8 of operating mechanism I6. Thiscircuit may be traced from the left-hand end of secondary winding 68through conductor I30, contact I05, contact 8|, contact I06, a conductorI33, winding I8, and conductor I32, to the righthand end of stationarywinding 68.

Energization of winding I8 will cause operating mechanism I6 to drivethe valve I5 in closing direction, thereby reducing the amount of heatsupplied to the space I0 by the furnace II. As long as the pointer 42 isdeflected to the right, thus indicating that the temperature in thespace I0 is above the desired value, the winding I8 is intermittentlyenergized, thereby reducing the supply of heat to the space I0.

Projections I0! and I08 are provided to limit the movement of contact 8|when the galvanometer is deflected to one of its extreme positions.Because of these projections, the contact 8| may not move to such aposition that it engages only contact I04 or contact I06. If these stopswere omitted, an extreme deflection of the galvanometer might causecontact ll to move to such a position, thereby preventing operation ofthe motor at a time when its operation was needed most. When contact itengages stop II! or I08, further movement of arm '2 in either directionis permitted by strain release spring at.

The operation of the system with both galvanometers 38 and 51 will nowbe considered. The function of the bridge circuit 43 and thegalvanometer 51 is to cut down the supply of fuel to the furnace whenthe resistance of the element 52 is such as to indicate that thetemperature of the air discharged from the furnace is becomingdangerously high.

The limiting value of the discharge duct temperature, above which it isdesired that the fuel supplied to the furnace be reduced regardless ofthe temperature in the space I0, may be determined for any particularsystem by means of the variable resistance 55. The system normallyoperates so that the discharge duct temperature is below that value. Atsuch a time the resistance of element 52 is lower than that required tobalance the bridge 43. The potential of output terminal 41 is thereforemore positive than that of output terminal 46 and a current is flowingfrom terminal 41 to terminal 46 through conductor 82, coil i5 andconductor 5|.

This current flowing through the coil 40 causes a deflection of pointer63 to the left of its central position. As the normal discharge ducttemperature is considerably below the limiting temperature, the bridge43 is normally considerably unbalanced, and the pointer 83 is deflectedconsiderably to the left of its central position. The pointer 63 istherefore, usually further to the left than the pointer 42, and thelatter pointer is the first one engaged by the arm 82 as it moves to theleft. The normal system operation is therefore that described in thepreceding paragraphs, where it was considered that the galvanometer 5'!was omitted.

If the temperature in the discharge duct l2 rises to its predeterminedlimiting value, the bridge 43 is balanced and the pointer 53 moves toits central position. On the next movement of the arm 82 to the left,the pointer G3 prevents any further opening movement of the valve l5. Ifthe pointer 42 is at the left of its center position, thus indicating acall for heat due to temperature in the space Hi, the plate 93 engagesthe pointer 63 before the plate 92 engages the pointer 42. The positionof the contact 9! now depends on the position of pointer 53 rather thanthe position of pointer 42 and since the pointer 53 is in its centerposition, neither winding l I nor 1 8 is energized. Therefore, noadditional supply of fuel to the burner is permitted. If, however, thetemperature in the space It is above its normal value so that pointer 42is deflected to the right, the pointer 63 has no effect on the operationof the mechanism and the switch contact Si is positionedin accordancewith the pointer 42 so as to engage contacts I05 and I" and energizewinding I! to drive the valve i5 towards closed position.

If the temperature in the discharge duct 12 increases above the limitingvalue which was set by the resistance 55, the resistance of element 52increases so that the potential of output terminal 41 becomes morenegative than that of terminal 46. A current therefore flows fromterminal 46 to terminal 4'! through conductor 6|, coil 60 and conductor62. This current flowing through coil causes a deflection of pointer 53to the right of its normal position. When the arm 82 next moves to theleft the plate I! engages the pointer 63, thereby setting the positionof contact 9! to the right of its normal position so that contacts I05and I05 both engage contact 9! when the contacts I05 and I" are lifted.This closes the energizing circuit described above for winding l8 ofvalve operating mechanism I6, so that the valve I 5 is driven towardsclosed position. Under these conditions, the valve is driven towardsclosed position regardless of the position of pointer 42. If the pointer42 happens to be further to theright than pointer 53, the valve would bedriven towards closed position anyway. If the pointer 42 was at the samerelative position as pointer 63 or further to the left than pointer 83,then the position reached by arm 82 would be controlled by pointer 63,and the valve [5 would be driven towards closed position as describedabove.

The plates 92 and 83 are made adjustable with respect to the arm 82 sothat variations in the neutral positions of the galvanometer pointersmay be taken care of.

While I have shown and described a preferred embodiment of my invention,modifications of this structure will readily occur to those skilled inthe art, and it should be understood that I wish to be limited only bythe scope of the appended claims.

I claim as my invention:

1. A condition control system, comprising in combination, conditionchanging means, a first electrical bridge circuit including an impedancevariable in accordance with the magnitude of a condition indicative ofthe need for operation of said condition changing means, a firstgalvanometer having a pointer and connected to said first bridge circuitso as to respond to an unbalance potential thereof, a second electricalbridge circuit including an impedance variable in accordance with themagnitude of a limiting condition, a second galvanometer having apointer and connected to said second bridge circuit so as to respond toan unbalance potential thereof, control means for said conditionchanging means, operating means variable in position to vary the effectof said control means on said condition changing means, and means forpositioning said operating means in accordance with the position of oneof said pointers, said positioning means selecting said one pointer inaccordance with the relative positions of said pointers.

2. A condition control system, comprising in combination, conditionchanging means, a first electrical bridge circuit, including animpedance variable in accordance with the magnitude of a conditionindicative of the need for operation oi. said condition changing means,means responsive to the unbalance of said first bridge circuit includinga first member variable in position in accordance therewith, a secondelectrical bridge circuit including an impedance variable in accordancewith the magnitude of a limiting condition, means responsive to theunbalance of said second bridge circuit including a second membervariable in position in accordance therewith, means for controlling saidcondition changing means, operating means variable in position to varythe efiect of said control means on said condition changing means, andmeans for positioning said operating means in accordance with theposition of one of said members, said lastclamping said pointers, a.positioner membeu movable along a path transverse to the paths ofmovement of said pointers, said positioner memher being stopped uponengagement with one of said clamped pointers, a switch arm positioned bysaid member, a pair of contacts selectively engageable with said arm,the particular contact engaged depending on the position of said arm,and a pair of control circuits each including one of said contacts.

4. A control system, comprising in combination, a pair of galvanometershaving movable pointers, said pointers moving in parallel planes, meansfor deflecting one of said pointers in accordance with the value of oneof a pair of controlling conditions, means for deflecting the otherpointer in accordance with the value of the other controlling condition,means for simultaneously clamping said pointers, a positioner membermovable along a path transverse to the paths of movement of saidpointers, said positioner member being stopped upon engagement with oneof said clamped pointers, and control means operable in accordance withthe position of said member.

5. In a control system, in combination, a pair of galvanometers havingmovable pointers, said' pointers moving in parallel planes, means forsimultaneously clamping said pointers, a positioner bar spanning thedistance between the planes of movement of said pointers, saidpositioner bar being movable along a path transverse to said planes,means for moving said bar along said path until it is stopped byengagement with one of said pointers, and control means operable inaccordance with the position in which said bar is stopped.

6. In a control system, in combination, a pair of galvanometers havingmovable pointers, said pointers moving in parallel planes, means forsimultaneously clamping said pointers, including a stationary barmounted between said pointers and extending parallel to their planes ofmovement, a pair of movable bars, each supported on the opposite side ofone of said pointers from said stationary bar, and means forsimultaneously moving said pair of bars towards said stationary bar soas to clamp said pointers therebetween, a positioner member movablealong a path transverse to the path or movement or said pointers. meansfor moving said member along said path until it is stopped by engagementwith one of said pointers, and control means operable in accordance withthe position in which said memher is stopped.

7. In a control system, in combination, a pair of galvanometers havingmovable pointers, said pointers moving in parallel planes, means forsimultaneously clamping said pointers, a positioner bar spanning thedistance between the planes of movement of said pointers, saidpositioner bar being movable along a path transverse to said planes, apair of members mounted on said bar, each member being adapted to engageone of said pointers, means for separately adjusting said members withrespect to said bar, means for moving said bar along said path until itis stopped by engagement of one of said members with its associatedpointer, and control means operable in accordance with the position inwhich said member is stopped.

8. In a condition control system, in combination, a first galvanometerhaving a first pointer, means for positioning said pointer in accordancewith the magnitude of a first condition, a second galvanometer having asecond pointer, means for positioning said second pointer in accordancewith the magnitude of a second condition, condition controlling means,operating means variable in position to vary the effect of saidcondition controlling means, and means for positioning said operatingmeans in accordance with the position of one of said pointers, saidpositioning means selecting said one pointer in accordance with therelative positions of said pointers.

9. A condition control system, comprising in combination, an electricalnetwork including a first condition responsive impedance, meansresponsive to a potential of said network indicative of the magnitude ofsaid condition including a first galvanometer having a pointer, a secondcondition responsive impedance in said network variable in accordancewith a limiting condition, means responsive to a potential of saidnetwork indicative of the magnitude of said limiting condition includinga second galvanometer having a pointer, condition controlling means,operatin means variable in position to vary the effect of said conditioncontrolling means, and means for positioning said operating means inaccordance with the position of one of said pointers, said positioningmeans selecting said one pointer in accordance with the relativepositions of said pointers.

10. In a control system, in combination, a pair of galvanometers havingmovable pointers, means for simultaneously clamping said pointers, apositioner bar movable along a path transverse to the paths of both saidpointers, means for moving said bar along its path until it is stoppedby engagement with one 01 said pointers, and control means operable inaccordance with the position in which said bar is stopped.

WILLIAM L. McGRATH.

